Field of the Invention
The present invention relates to a thin film transistor (in general, called "TFT") and a method of manufacturing the same, and particularly to a method of forming source and drain regions in a thin film transistor.
In recent years, there have been known active matrix liquid-crystal display unit using a thin film transistor. FIGS. 2A to 2D show a process of manufacturing a general thin film transistor. First, a silicon oxide film or silicon nitride is formed on a glass substrate 201 as a first coating film 202. A Corning 7059 glass or the like is used as a glass substrate. After the formation of the first coating film 202, a silicon semiconductor film which forms an active layer is formed on the first coating film 202. An amorphous silicon film is usually formed through the plasma CVD technique or low pressure thermal CVD technique, and thereafter the amorphous silicon film is crystallized by heating or the application of laser beam. Then, a silicon film subjected to a crystal property (hereinafter referred to as "crystalline silicon film") is patterned to thereby form an active layer 203. (FIG. 2A)
After the formation of the active layer 203, a silicon oxide film is formed as a gate insulating film 204 through the plasma CVD technique or the sputtering technique. Then, a gate electrode 205 is formed of material mainly containing metal or semiconductor. After the formation of the gate electrode 205, impurity ions are injected thereinto so as to form a source region 207 as well as a drain region 209. This process is executed using the gate electrode 205 as a mask. As the ions injected, P (phosphorus) is used in the manufacture of an n-channel thin film transistor, whereas B (boron) is used in the manufacture of a p-channel thin film transistor. Also, a channel formation region 208 is formed simultaneously during this process. (FIG. 2B)
After the formation of the source region 207 and the drain region 209 as well as the channel formation region 208, the source region 207 and the drain region 209 are recrystallized by application of a laser beam or an infrared ray, and the impurity ions injected into those region are activated. The recrystallization of the source region 207 and the drain region 209 are made because the source region 207 and the drain region 209 have been made amorphous by the bombardment of injected ions at the time of the preceding ion injection.
The above-mentioned recrystallization and activation of the source and drain regions may be performed by heating. However, in the case of heating, its effect could not be obtained without heating at temperature of 700.degree. C. or higher (preferably 800.degree. C. or higher). Taking the heat-resistivity of a glass substrate (a substrate made of Corning 7059 glass must be dealt with at 600.degree. C. or lower) into account, such a heat treatment is improper.
Subsequently, an interlayer insulating film 211 is formed of silicon oxide or other insulating materials. Further, after forming contact holes, a source electrode 212 and a drain electrode 213 are formed of a proper metallic material.
The thin film transistor manufactured through the foregoing processes suffers from such a problem that its characteristics are deteriorated or largely dispersed. This problem results from the fact that defects concentrate in the vicinity of interfaces between the source region 207 and the channel formation region 208 and between the drain region 209 and the channel formation region 208.
In other words, the source region 207 and the drain region 209, which have been made amorphous by the injection of ions in the process of FIG. 2B, are recrystallized by the application of a laser beam in the process of FIG. 2C, during which the channel formation region 208 remains crystalline. Therefore, the crystallization of the source and drain regions, which progresses by the application of a laser beam, stops at the interfaces between the source and drain regions and the channel formation region having the crystal property from the first. As a result, a large number of defects resulting from mismatching of lattices are produced in the vicinity of the interfaces between the source and drain regions and the channel formation region. The existence of those defects makes not only the characteristics dispersed and unstable but also an off-state current increase.
As a manner of solving the foregoing problem, it has been found that the recrystallization of the source and drain regions and the activation of the impurity ions are performed at a temperature of 700.degree. C. or higher, preferably 800.degree. C. or higher. If the recrystallization of the source and drain regions and the activation of the impurity ions are performed at a temperature of 700.degree. C. or higher, preferably 800.degree. C. or higher, energy is also applied to the channel formation region 208. Hence, mismatching of lattices produced in the vicinity of the interfaces between the source and drain regions and the channel formation region can be released, as a result of which the defects can be prevented from concentrating in the vicinity of the interfaces between the source and drain regions and the channel formation region.
However, in order that processes for the recrystallization of the source and drain regions and the activation of impurity ions injected are performed by a process of heating at 700.degree. C. or higher, a substrate capable of resisting a temperature of 700.degree. C. or higher must be used. However, such a substrate is expensive, resulting in a large obstacle to the use of the thin film transistor in a liquid-crystal display apparatus. In other words, in the use of an inexpensive glass substrate having a heat-resistant temperature of 600.degree. C. or lower, the processes for the recrystallization of the source and drain regions and the activation of impurity ions cannot be realized by heating for all practical purposes.